Eye disease, arthritis, folliculitis, and thrombophlebitis were more common among males, and erythema nodosum among females, in 297 patients with Behcet syndrome (BS) at their first visit despite an identical disease duration. Younger males and females (age of onset 24 years or less) had a higher prevalence of eye disease and total clinical activity than did the older patients (age of onset 25 years or more). Among the 51 patients followed up for 52 months (SD 7 months) the total clinical activity became significantly less (p less than 0.05) in the whole group at the end of this period. This was particularly true for older females. While male sex and a younger age of onset are associated with more severe disease in BS, overall, the syndrome ran an improving or stable course over 4 1/2 years.